This invention relates to an essentially lead free glass. The invention also relates to a glass tube made from the essentially lead free glass. The lead free glass has properties making it particularly suitable for use in tubes of electric lamps. Therefore the glass may be used in the manufacture of light bulbs.
Nowadays glass tubes used in the manufacture of electric lamps are usually made from either a soda-lime glass or a lead glass. The lead glass contains lead oxide. Both lead oxide dust and lead vapor from melting lead oxide can cause dangerous air pollution harmful to the environment and to human beings.
Lead oxide acts as a fluxing agent and gives lead glass a lower softening poet than soda-lime glass. It is desirable for glass that is used in making tubes for electric lamps to have this lower song point but without containing lead oxide,
For making such a glass suitable for use in electric lamp production, we have substituted lithium carbonate for the lead oxide. Lithium carbonate is also a fluxing agent. Because of this it allows an increase in the melting rate of the glass by lowering its viscosity. It also leads to a lower seed (bubble) count, a lower thermal expansion coefficient and a higher chemical durability of the glass as well.
We have also found that when cerium oxide and titanium dioxide are mixed with the essentially lead free glass, a glass may be made that is capable of absorbing substantially all ultraviolet (UV) radiation having a wavelength shorter than 320 nm.
It has been known that cerium oxide may be used as a decolouriser and a refining agent. It also acts as a flux in glass making and helps clear gas by reducing seeding and bubble formation in molten glass. In addition, the use of cerium oxide has also been reported to increase the life expectancy of the melting electrodes used in electric furnaces.
Cerium oxide may have its performance complemented by the addition of sodium nitrate, which causes a chemical reaction with the cerium oxide.
The presence of cerium oxide, with or without the addition of sodium nitrate, does not increase the absorption of the visible spectrum of light and light transmittance at wavelengths greater than 320 nm. is particularly satisfactory for the manufacture of electric lamps.
An object of at least one preferred embodiment of the present invention is the provision of an essentially lead free glass suitable for the manufacture of tubes for electric lamps.
Another object of at least a preferred embodiment of the invention is such an essentially lead free glass tube. This is achieved by replacing the potentially harms lead oxide present in lead glass.
Another object of at least a preferred embodiment of the invention is to provide an essentially lead free glass that absorbs substantially all UV radiation having a wavelength shorter than 320 nm. while, preferably, increasing the efficiency of transmission of visible light. These features are also desirable in glass tubes used in the manufacture of electric lamps.
In a first aspect, the present invention broadly consists in an essentially lead free glass comprising:
The lead free glass will typically comprise 65.0-75.0% SiO2. It may contain one or more, and preferably contains all of the following: 2.0-4.0% Al2O3, 1.0-3.0% B2O3, 3.0-5.0% BaO, a total of 6.0-9.0% MgO and/or CaO, 6.0-9.0% Na2O, 3.0-5.0% K2O, 1.0-3.0% Li2O, 0.1-0.3% CeO2, optionally about 0.01% Fe2O3 and 0.05-0.15% TiO2.
In a second aspect, the present invention broadly consists in a glass tube made from an essentially lead free glass as defined above.
In a third aspect, the present invention broadly consists in an electric lamp having its tube (which term includes xe2x80x9cbulbxe2x80x9d) made from the essentially lead fee glass as defined above.
In the specification, unless otherwise stated, percentages are by weight,